The present invention relates to a cathode for an electron tube and, more particularly, to a thermal electron emitting cathode having an enhanced life span for use in an electron tube such as a cathode-ray tube or image pickup tube.
In general, referring to FIG. 1 the cathode comprises a disk-like base metal 2, a cylindrical sleeve 3 which is fitted to the bottom surface of the base metal 2 for support and provided with a heater 4 located inside the sleeve for heating the cathode, and an electron-emitting material layer 1 coating the upper surface of the base metal 2.
Here, the electron-emitting material layer 1 is generally an alkaline earth metal oxide having barium oxide as a major component, preferably, a ternary metal oxide expressed as (Ba,Sr,Ca)O.
Such an electron emitting material layer 1 is formed as follows. First, a solution is prepared by dissolving a mixed powder of barium carbonate, strontium carbonate and calcium carbonate in an organic solvent such as nitrocellulose or the like, and then a carbonate salt coating is formed by spraying or electrolytically depositing the prepared solution on a base metal of a cathode used in an electron tube. Afterwards, the carbonate salt coating layer is heated to 1000.degree. C. by heater 4 in a vacuum during which the carbonate salt is converted to an oxide, e.g. barium carbonate is converted to barium oxide. Such a cathode is called an "oxide cathode" because the carbonate salt is converted to an oxide in the vacuum process by the high temperature. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 .uparw. (1)
During cathode operation, the barium oxide reacts with a reducing agent, Si or Mg, contained in the base metal at the interface where the base metal and the layer of the electron-emitting substance contact, to form free barium as shown in the following formulas. EQU BaO+Mg.fwdarw.MgO+Ba.uparw. (2) EQU BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw. (3)
The the free Ba contributes to electron emission. Furthermore, MgO, Ba.sub.2 SiO.sub.4 or the like are formed in the interface between the electron-emitting substance and the base metal, and serve as a barrier called an "intermediate layer," to prevent the Mg or Si from diffusing. Accordingly, the intermediate layer inhibits the generation of free Ba. Consequently, the intermediate layer contributes to shortening the life span of the cathode. There is another disadvantage in that the high resistance of the intermediate layer prevents the flow of current for emitting electrons and thus limits current density.
Along with popular trends toward higher definition and larger screens for televisions and other devices using cathode-ray tubes, there has been an increasing need for cathodes with high current densities and longer life spans. However, conventional oxide cathodes are not capable of satisfying this need due to the aforementioned disadvantages with respect to performance and life span.
An impregnated cathode is known for its high current density and long life span, but the manufacturing process therefor is complex and its operating temperature is over 1100.degree. C., that is, about 300.degree. C. or 400.degree. C. higher than that of oxide cathodes. Accordingly, since it is required that the cathode be made of a material with a much higher melting point which is expensive, its practical use is impeded.
Thus, a great deal of research has gone into lengthening the life span of a conventional oxide cathode with a high degree of practicality. For example, U.S. Pat. No. 4,797,593 owned by Mitsubishi discloses a technique for improving the life span of a cathode by dispersing a rare-earth metal oxide such as Sc.sub.2 O.sub.3 or Y.sub.2 O.sub.3 into a conventional ternary carbonate. Also, U.S. Pat. No. 5,146,131 discloses including Eu.sub.2 O.sub.3 in an electron-emitting substance to lengthen cathode life span.
The cathodes containing rare-earth metals have enhanced life spans because the rare-earth metal inhibits an intermediate layer from being formed and free Ba from being evaporated. However, the amount of electron emission of the cathode tends to drop off suddenly after a certain period of operation time because the rare-earth metal accelerates sintering of oxides at the operating temperature of the cathode. Thus, the oxide is charred hardened, which results in a decrease of reaction sites for the reducing agent, reducing the quality of emitted electrons. Moreover, because the cathodes have poor cut off drift characteristics and do not have complete interchangeability with a conventional oxide cathode during the manufacturing process, a cathode activation process for ensuring a steady and abundant emission of thermal electrons is required.